A hypereutectic aluminum-silicon alloy containing silicon with the composition at a eutectic point or more, i.e. 12.6% by mass or more of aluminum (Al)-silicon (Si) alloy has a small linear thermal expansion coefficient and also has excellent wear resistance. This is because silicon content with the composition at a eutectic point or more enables formation of a primary crystal Si during solidification, and these are characteristics which are not obtained in a hypoeutectic aluminum-silicon alloy in which the silicon content is less than the composition at a eutectic point (i.e. less than 12.6% by mass) and a primary crystal Al is formed.
Particularly, when the silicon content is within a range of 20.0% by mass to 30.0% by mass, a sufficient amount of a primary crystal Si is obtained and thus the linear thermal expansion coefficient more decreases to a linear thermal expansion coefficient which is almost the same as that of copper. Wear resistance is also significantly improved and, furthermore, the obtained alloy has high thermal conductivity.
Therefore, the hypereutectic aluminum-silicon alloy having the silicon content of 20.0% by mass to 30.0% by mass is expected to have a wide application field, including applications such as a substrate for semiconductor device, including a wiring made of metal such as copper on a surface thereof, and various housings (casings).
However, the hypereutectic aluminum-silicon alloy has a problem that it is difficult to obtain a desired shape by secondary forming process because of poor workability after casting.
Therefore, a die casting method has been proposed, as a method for casting a hypereutectic aluminum-silicon alloy having poor workability into a desired shape.
The die casting method is capable of easily obtaining a final shape or a shape close to a final shape, and has an advantage that there is no need to subject the obtained die-cast member to the steps of grinding and polishing. Even if these steps are performed, it is easy to perform by slight machining.
However, it is believed that the silicon content of more than 17% leads to poor fluidity of a molten metal and that the hypereutectic aluminum-silicon alloy having the silicon content of 20.0% by mass to 30.0% by mass exhibits considerably poor fluidity of the molten metal, and thus it is difficult to perform die casting by a conventional die casting apparatus even in the case of not only a thin-wall member but also a usual member. Therefore, die casting was scarcely performed.
Namely, even if the hypereutectic aluminum-silicon alloy containing 20.0% by mass to 30% by mass of silicon is used as a mother alloy (silicon source) so as to obtain a die-cast member of an aluminum-silicon alloy with lower silicon content, a die-cast member of the hypereutectic aluminum-silicon alloy containing 20.0% by mass to 30% by mass of silicon scarcely existed as a practical material.
This becomes apparent from the fact that Patent Document 1 discloses a high thermal conductivity alloy for pressure casting (die casting), containing 5 to 16% of silicon, and also mentions that fluidity becomes maximum when the silicon content is about 15% while castability deteriorates when the silicon content becomes 16% or more.
Regarding the range where the silicon content is less than 20.0% by mass, for example, Patent Document 2 discloses a method in which a molten metal is poured into a sleeve and the molten metal is held at a temperature range between the crystallization temperature of a primary crystal Si and the eutectic temperature, followed by injecting molding to obtain a die-cast member, so as to obtain a wear-resistant member made of an aluminum-silicon alloy having the silicon content of 14 to 17% by weight.
In a range where the silicon content is close to a range of 20.0% by mass to 30.0% by mass, for example, Patent Document 3 discloses a method in which, so as to impart vibration-absorbing property by crystallizing a large primary crystal Si, a molten metal of an aluminum-silicon alloy containing 20 to 33% of silicon is held at a temperature lower than the liquidus temperature of the alloy for a comparatively long time, for example, one hour, and then die casting is performed in a state where the molten metal containing a large amount of the crystallized silicon.
Regarding the range where the silicon content is more than 30%, for example, Patent Document 4 discloses a method for producing a heat dissipation member using a die casting method, in which a molten metal of an aluminum-silicon alloy at 980° C., which is prepared by mixing 37% of silicon with aluminum as the balance and melting the mixture by high frequency induction melting in the Ar atmosphere, was poured in a die-cast mold, followed by compression forming at 920° C. for 3 seconds under 15 MPa.    Patent Document 1: JP 2001-316748 A    Patent Document 2: JP 11-226723 A    Patent Document 3: JP 58-16038 A    Patent Document 4: JP 2001-288526 A